Entangled photon states are special quantum states of light which have been shown to be useful for various applications such as quantum key distribution and quantum metrology. This invention is related to the creation of entangled photon states in a robust, practical, and controllable manner in such a way as to be conveniently measurable. Entangled light can be generated using various nonlinear processes including those in nonlinear crystals, such a periodically poled lithium niobate, as well as using the third order nonlinearity in fiber. The use of fiber is beneficial because it is often desired to inject the entangled photons into fiber in order to propagate them over long distances. By generating the entangled photons directly in fiber one can avoid coupling losses. Other benefits, such as high spatial mode purity and the potential for simple manufacturing, are also realized. We note that entangled light is generated by properly combining quantum correlated light beams, and thus the invention herein is also applicable for generating quantum correlated light beams. However, correlated beams are generally easier to prepare and measure, thus some features of the invention are primarily applicable to entangled states.
Some schemes for realizing entanglement using the nonlinearity of fiber have been specified by the same inventive entity as the present invention in a U.S. Pat. No. 6,897,434 by Kumar et. al. Later work was published which used a modified design in order to make the system more robust and easier to align. Further development of the method was performed in a US Patent Application Pub. No. 20090268276, where certain practical issues especially as pertains to designing the entangled light source to allow for simplified alignment of the downstream measurement apparatus were considered.
It is desirable to engineer an entangled photon source which is simple to align and for which the alignment of the source and the subsequent detection apparatus could be easily automated. For polarization entangled light, the detection apparatus can be a polarization analyzer, of which one implementation is shown in FIG. 1. The detection apparatus detects the paired photons, one photon traditionally called the signal and the other the idler. The signal is input to one polarization analyzer 10 and the idler to another 11. They each contain a series of optical waveplates for causing polarization transformations of the incoming light, in this case a half wave plate 12,13 and a quarter waveplate 14,15 although other types of polarization analyzers can use other components such as variable waveplates and have more or fewer components. The waveplates are mounted on rotatable stages. In FIG. 1 each analyzer also has a rotatable polarizer 16, 17. The polarizers act as polarization projection devices, and can also be realized with polarization beam splitters. The photons exiting the polarization analyzers are detected with single photon detectors 18, 19 at which point the output from each detector is counted and correlated in a processor 20. An optional variable waveplate 21 that can be realized with a liquid crystal phase retarder and which is also on a rotatable platform is inserted before one of the rotatable polarizers. It is the nature of entangled sources that interference can occur in the correlations of the detectors as a function of the angle of the rotatable polarizer, even though the statistics of the singles counts are not polarization dependent. The quality of this interference can be recorded as a two-photon interference (TPI) fringe. In order to record this TPI fringe, the various polarization transformations prior to the polarizer need to be set correctly.
Polarization entangled light is sometimes difficult to measure because the polarization rotations that take place in the fiber connecting the entangled source to the measurement device need to be properly accounted for. There are three independent variables that control polarization (to convert any input state of polarization to any output state). Polarization entangled light thus has more degrees of freedom to account for than time-bin entangled light which typically only needs to control optical phase. However, entanglement in the polarization mode can be useful for several reasons including the usually lower cost and lower loss of polarization control devices as compared to the devices needed to manipulate relative phase. Additionally, if one has control over the polarization then hyper-entangled sources entangled in both polarization as well as other modes are possible. Thus this work focuses on polarization entanglement. Since polarization is the harder parameter to control the methods are also suitable to the generation of hyper-entanglement or for systems that need to be able to generate multiple kinds of entanglement that include polarization entanglement.
Since polarization entangled light is effectively depolarized, the photon counts from a particular detector 18, 19 are not a function of the setting of the polarization analyzers 10, 11. However, the analyzer must be set properly in order to make a desired measurement since the correlations between the detectors are a function of the settings of the polarization analyzers. The settings may be relatively easy to determine when using an apparatus that generates entanglement in free-space. In such a case, as in U.S. Pat. No. 6,424,665 by P. G. Kwiat et al., the two orthogonal polarization modes which are the constituent components of the entangled light exit the source, typically at polarizations called H and V, which can be referenced to the physical axis of the laboratory and correspond to horizontal and vertical polarizations. For this reason the polarization analyzer used in U.S. Pat. No. 6,424,665 is a simple half-wave plate followed by a polarizer which is equivalent to a rotatable polarizer. The H and V axis are clearly defined in physical space. There is a relative phase term between the H and V axis that must be set, producing an entangled state of |H|H+eiφ|V|V, but that phase can be set, for instance, via changing the phase between the H and V axis on the pump wave. This phase typically does not drift considerably over time so the setting of the phase is a rare event.
Adjusting the polarization analyzer to the correct setting becomes more difficult if the entangled light propagates through fiber—particularly if both the signal and idler propagate through different fibers as will generally be the case. This is because there is an unknown polarization rotation due to birefringence in the fiber. Physical space can no longer be used as a reference and the polarization rotation has multiple degrees of freedom. One can not easily set the polarization analyzer using the entangled light directly. This is because the entangled light is not polarized so changing the analyzer settings has no effect on the singles counts. One can search for the settings that lead to the desired coincidence count performance, but this is difficult to do due to the dimensionality of the system and the fact that coincidence counts are relatively rare events. Coincidence counts are rare because losses reduce co-incidences in a quadratic way and entangled light sources typically generate much less than one photon pair per measurement interval in order to reduce multi-photon pair generation events.
It is beneficial if a polarized high-intensity signal is used to aid in alignment. This allows one to produce many alignment photons per each measurement interval whereas the entangled state generation typically generates much less than one photon per measurement interval. A higher photon rate allows for faster measurement speed and therefore faster alignment. The speed at which the system can be aligned is particularly important in fiber, since the birefringence in fiber changes as a function of time. Thus, being able to quickly determine the correct settings for the polarization analyzer, or to periodically readjust the setting, is of importance. Also, it is generally easier to use local singles counts (optical intensity) to set the polarization analyzers, such as using the singles counts from the signal single photon detector 18 as the feedback signal to set the polarization transformations in the signal polarization analyzer 10. Keep in mind that in an actual application the signal and idler photons may be detected in different locations.
A recent US patent application Pub. No. 20090268276 by the same inventive entity describes an invention which allows the polarization analyzers to be set in a two step process. First a polarized alignment laser is used to generate photons at the signal and idler wavelengths with a particular polarization with respect to the constituent orthogonally polarized pulses that are combined to create the entanglement. This allows for each polarization analyzer to be set to, say, minimize this polarized light signal passing through the polarizer thereby aligning two degrees of freedom of the polarization rotation. After this adjustment the entangled source is set to produce entangled light while the rotatable polarizers 16, 17 are rotated by an angle, typically 45 degrees. The phase of the variable waveplate 21, which had its angular position set so that its optical axis is either parallel or perpendicular to the polarized light, is then adjusted in order to maximize the correlations between the signal and idler photons. In this way only one parameter, the phase of the variable waveplate, is adjusted using correlations. Other types of polarization analyzers could be used, with the internal polarized alignment signal of the invention used as a basic tool used to help align the analyzer. The entangled photon source architectures disclosed in US Patent Application Pub. No. 20090268276 is focused on the use of Faraday mirrors in order to maintain a stable polarization.
An architecture for generating entangled photons from a fiber source using Sagnac loops, also known as Sagnac interferometers, was described in U.S. Pat. No. 6,897,434 by Kumar et. al., fully incorporated herein by reference. This method may have some advantages including typically lower insertion loss which is important because loss lowers the correlated entangled photon detection rate in a quadratic way. However, in its original form the architecture requires the manual adjustment of an in-loop polarization controller and uses an undesirable amount of free-space optical components. What is desired is an improved design that could be more easily automated and manufactured thereby making it more practical.
Although the prior art represents fairly practical designs, what is desired is a system that can be easily aligned and whose alignment procedure can be easily automated, which also keeps the cost of the components as low as possible. For instance, the tunable alignment laser used in US patent application #20090268276 is a relatively expensive component which would be beneficial to eliminate. The invention herein makes use of more convenient broad-band sources such as light emitting diodes or the amplified spontaneous emission from an optical amplifier in order to generate an alignment signal. This broad-band source can generate alignment signals at multiple signal/idler wavelengths simultaneously, allowing one alignment source to be used to align multiple detection apparatuses. Additionally, methods are described which allow for the generation of alignment signals with two different non-orthogonal polarizations. By using two different alignment polarizations, the polarization transformations of the polarization analyzers can be completely specified without requiring the use of coincidence counting. Other desired features pertain to reducing the internal losses of the system, and maintaining better control over the generated state so that an entangled state can be both generated and the downstream measurement apparatus subsequently easily aligned to it with high precision. In some cases, the alignment of the downstream measurement apparatus can be done using only singles counts as a feedback signal, as opposed to the more fragile coincidence count measurements. Sometimes it might be useful to be able to generate various states including correlated photons or non-maximally entangled states, and some embodiments of this invention allow such states to be generated if desired.